1. FIELD OF THE INVENTION
The present invention relates to a method for descaling a hot-rolled stainless steel strip and more particularly, to a descaling method which improves the descaling step for removing scale from the surface of a stainless steel strip, while improving the state of the strip surface after the descaling.
2. DESCRIPTION OF THE PRIOR ART
In general, a stainless steel strip is more liable to exhibit work hardening as compared with ordinary steel strips and, hence, tends to increase the load during cold rolling. In order to reduce the load in cold rolling, it has been proposed and carried out to effect a continuous annealing followed by descaling on hot-rolled stainless steel strips by an equipment called an AP line (Annealing and Pickling Line). As to the detail of this treatment, a reference is to be made to, for example, Iron and Steel Handbook III (1), Maruzen, May 15, 1980, particularly "Basics of Rolling of Steel strips", pp 699-700.
FIG. 5 shows the outline of the conventional AP line. This AP line has a pay-off reel 1, an entry-side shear 2, a welder 3 and an entry-side looper 4. Numeral 5 designates an annealing furnace having a heating section 6 and a cooling section 7. The heating section 6 includes a preheating zone, heating zone and a soaking zone. The AP line further has a shot blast 8, a plurality of pickling tanks 9, 10, 11 including a sulfuric acid tank, nitro-fluoric acid tank and a nitric acid tank, a rinsing device 12, a drier 13, a delivery-side looper 14, a shear 15 and a tension reel 16.
According to a current practice, a scale breaker including a roll bender is disposed on the entry-side of the shot blast 8, in order to enhance the descaling effect. It is also attempted to provide a grinding brush on the delivery-side of the shot blast 8.
In the operation of this AP line, a hot-rolled stainless steel strip S uncoiled from a pay-off reel 1 is made to pass through the entry-side shear 2 which cuts and trims the leading end or trailing end of the stainless steel sheet S. The trimmed end of the stainless steel sheet S is jointed to a preceding coil or a subsequent coil by means of a welder 3. The stainless steel sheet S is then fed through the entry-side looper 4 into the annealing furnace 5 so as to be heat-treated in this furnace. More specifically, in the heating section 6 of the annealing furnace 5, the stainless steel strip S is supported in the form of a catenary by means of asbestos rolls 71 and is heat-treated by a direct contact of a flame formed by a burner. The sheet S is then moved into the cooling section 7 where it is cooled by air or water. The steel strip S is then mechanically descaled by, for example, a shot blast and is then made to pass through the pickling tanks including a nitro-fluoric acid tank so as to be chemically descaled and passivated. The steel sheet S is then made to pass through the rinsing device 12 in which the surfaces of the steel sheet S are cleaned by brushing and spraying, and is further introduced into the drier to be dried. The steel strip is then advanced through the delivery-side looper 14 to the shear 15 so as to be cut at a predetermined length. The steel sheet S is then coiled into the tension reel 16.
In this conventional AP line, annealing is effected by direct contact of the burner flame, so that the oxide scale of about 5 .mu.m thick, which has been formed on the surface of the steel strip S during hot rolling, grows to have finer structure and a greater thickness of 10 to 30 .mu.m. The oxide scale thus grown up on the stainless steel is extremely difficult to remove as compared with the case of ordinary steel strips, because the structure of the scale is very fine. This is the reason why both mechanical descaling by a shot blast and chemical descaling such as pickling by a plurality of acid tanks are necessary. The shot blast is indispensable, particularly when the stainless steel strip has been hot rolled. In addition, the pickling has to be done for a considerably long time with a pickling solution having a very high concentration, e.g., 20 wt% or so, of a strong acid such as sulfuric acid, nitro-fluoric acid (mixture of nitric acid and fluoric acid) or nitric acid. Consequently, a very long time is required for descaling, making it difficult to enhance the yield of the product.
It is also to be pointed out that the surfaces of the steel strip are undesirably roughened as a result of collision by shot grains during the shot blasting. In addition, pickling by a strong acid such as nitric acid undesirably enhances corrosion at grain boundaries. Namely, annealing of a stainless steel strip causes a chromium depleted zone in grain boundaries so as to allow a heavy corrosion of the grain boundaries by the pickling. Nevertheless, the use of nitro-fluoric acid as the pickling liquid for austenitic stainless steel strip is indispensable because this acid has quite a strong descaling effect, as a cost of the grain boundary corrosion.
Presence of minute convexities and concavities, i.e., roughness, formed as a result of shot blasting or grain boundary corrosion in the stainless steel strip surface allows generation of partial luster defect in the subsequent cold rolling. Generation of this defect is considered to be attributable to "scab" of minute projections on the sheet surface during the cold rolling.
Hitherto, various proposals have been made for reducing the requirement for descaling thereby to overcome the above-described problems. For instance, Japanese Pat. Laid-Open No. 49-135824 discloses a method in which an agent mainly composed of an alkaline metal salt and/or boric acid is applied to a hot-rolled ordinary steel strip so as to fuse the oxide film with said salt, whereby a steel strip easy to descale is obtained. According to this method, it is possible to reduce fluctuations in pickling time attributable to variations in the state of oxide scale, thus shortening the pickling time. On the other hand, Japanese Pat. Laid-Open No. 61-153291 discloses a method in which an aqueous solution of an alkaline metal halide is applied to the surface of a hot-rolled stainless steel sheet, followed by annealing, cooling and pickling, whereby the stainless steel strip is descaled at a high efficiency.
The method disclosed in Japanese Pat. Laid-Open No. 49-135824, however, is intended for use on ordinary steel strips which do not essentially require annealing and descaling prior to pickling. In addition, this method is not effective in removing spinel-type oxide film which is peculiar to stainless steel strips.
The method disclosed in Japanese Pat. Laid-Open No. 61-153291 exhibits an appreciable effect in removing persistent oxide scale formed on stainless steel strip. This effect, however, is not so large as to enable omission of shot blasting. In addition, this method utilizes a strong acid solution, in particular nitro-fluoric acid, as the pickling liquid, failing to meet the demand for improving the state of surface of the steel strip. Thus, the aforementioned problems of the prior art still remain unsolved.